The present invention relates to a process for the replication of existing surface textures into materials that cannot conveniently be embossed, deformed or optically patterned.
Surface textures have been extensively used to fabricate a variety of optical elements such as diffraction gratings, holograms and thin film elements for integrated optics. Other applications for surface textures occur in the construction of optically enhanced solar cells. Optically enhanced solar cells contain a periodic or random surface texture which increases the efficiency for absorption of light within the cell, see e.g., H. Deckman, H. Witzke, C. Wronski, E. Yablonovitch, Appl. Phys. Lett. 42. 968 (1983) and H. W. Deckman, C. R. Wronski and H. Witzke, J. Vac. Sci. Technol. A1, 578 (1983).
Replication techniques used to produce textured surfaces are of three principal types, optical, physical and metallic coating.
Many electronic devices and optical elements are fabricated of materials of carefully selected electronic and optical properties. These materials, in many cases, cannot be embossed or deformed to form a desired surface texture. It is therefore necessary to define a lithographic mask on the surface of each device or element which will protect certain portions of the surface from attack by etchants. This mask, when removed after etching, will reveal the texture in the surface of the device or optical element which was previously defined in the mask.
Optical replication techniques usually define a lithographic mask on the surface of the device or optical element by coating the part with a thin layer of polymer that is sensitive to U.V. light or to other ionizing radiation. The polymer is then exposed by an optical system which contains the desired pattern. Depending on the exposure, portions of the polymer can then be dissolved by solvents leaving a patterned lithographic mask on the surface which is a replica of the pattern contained in the optical system. The practical limitations of this process are reached when texture sizes lie below 0.5 .mu.m or when the surface of the device is curved. Furthermore, the pattern must be photolithographically defined on each individual device.
Physical replication techniques which have been previously used involve plastic deformation of some part of the substrate material in a molding, embossing, solution casting or polymerization process. Physical replication processes have been extensively used to copy diffraction gratings and are described by White et al., U.S. Pat. No. 2,464,739; G. D. Dew and L. A. Sayre, Proc. Ray. Soc. A 207, 278 (1951); G. D. Dew, J. Sci. Instrum. 33, 348 (1956); and I. D. Torbin and A. M. Nizihin, Optical Technology 40, 192 (1973). These processes involve contacting a master pattern with an organic liquid material which solidifies to produce a copy of the original surface. The solidification occurs because of cooling, solvent loss, or polymer formation; in molding, solution-casting or polymerization processes, respectively. In all cases the solidified material becomes part of a substrate with a copy of the original pattern on its surface. As such, the substrate surface is created during pattern transfer by plastic deformation of a liquid like material in a physical replication process. Optical elements such as fly's eye lenses, diffraction gratings, spherical and aspherical mirrors and holograms have been fabricated using physical replication techniques described in SPIE Vol. 115, Advances in Replicated and Plastic Optics, (1977) and by T. D. Torbin and A. M. Nizihin, Optical Technology 40, 192 (1973). In all of these techniques patterns are transferred into an organic polymerizing media by some type of physical replication process. Physical replication techniques are extensively used to copy audio and video disks from metallic master patterns into polymerizing materials and are described in RCA Review 39 (1978). All physical replication methods rely on plastic deformation of a material which is eventually incorporated into the final substrate.
Metallic replicas are formed by metallic plating of a master pattern to a thickness such that the deposit can be peeled off the master pattern. The metallic replica is a negative of the original pattern and is usually laminated onto the surface of a pre-existing substrate. Replicas can be formed without using plastic deformation by metallic coating techniques which are described in RCA Review 39 (1978). Using either physical replication or metallic plating methods, the texture on the master is directly reproduced and cannot be substantially altered in a predetermined manner during replication.
The present invention discloses a method for replicating microstructured patterns into pre-existing organic or inorganic substrates using an intermediate lithographic transfer mask. The invention allows patterns to be transferred from masters into substrates which could not be patterned with previously available techniques. Previous replication techniques copy patterns using processes which require either deformation of a substrate material or metallic plating methods or optical patterning of each individual optically flat substrate. The present invention does not require the substrate material to be flat or to be plastically deformed or to be metallic for the transfer and hence can be easily used on substrate materials such as glass, alumina and other refractory oxides, and semiconductors.
The present invention provides an improved process for replication which can be used with pre-existing organic and inorganic substrates. The method is not limited by choice of substrate into which the pattern is transferred as is the case with physical replication and plating methods.
The present invention also provides an improved process for replication in which the shape of the surface texture can be altered in a predetermined manner during the replication process. The flexibility of altering the shape of the surface texture comes from use of an intermediate lithographic mask to transfer the pattern from master to substrate.
The present invention also provides an improved process for replication in which a texture on a flat master can be transferred to a curved substrate. Alternatively a texture may be transferred from a curved master to a flat substrate or from a curved master to a curved substrate. This transfer between flat and curved surfaces is made possible by the flexibility of the intermediate lithographic transfer mask.
The present invention also provides an improved process for replication which can rapidly copy large as well as small area masters.
The present invention also provides an improved process for replication capable of producing a large number of copies from a single master pattern.
The present invention also provides an improved process for replication which does not require use of an optical system to transfer the pattern from master to replica.